1. Field of the Invention
The present invention relates to a semiconductor laser, especially to a semiconductor laser for communication systems that has a short laser cavity length and is suited for a wavelength-tunable operation in a wide wavelength range, and to an optical module of the same.
2. Description of the Related Art
It is known that, in the tunable distributed reflection type semiconductor laser in which an active waveguide having a function of light amplification and a waveguide reflector that has a diffraction grating and adjusts an emission wavelength by varying a reflection wavelength range are connected monolithically in a traveling direction of light, hopping of the laser axial mode occurs as the wavelength varies. This phenomenon can be explained by that a mode hopping spacing Δλ of the distributed reflection type laser without a phase adjustment region and an active region length La are linked to each other by a relationship:Δλ=λ2/2naLa
and the amount of phase change equal to 2π may occur depending on the wavelength change Δλ. Here, na denotes the refractive index of a laser medium, and λ denotes an oscillation wavelength. A semiconductor laser of this type is described in Technical Digest, Paper TuB4, 17th International Semiconductor Laser Conference where La is about 410 μm, and the longitudinal-mode hopping occurs periodically at Δλ=0.8 nm as calculated from the above equation. Therefore, the oscillation wavelength is controlled to be in the vicinity of the Bragg wavelength where mode hopping is minimized by use of a complicated mode-stabilizing circuit. Further, at the same time, the oscillation wavelength is separately controlled to coincide with a desired standardization wavelength grid that is used in high-density wavelength division multiplexing communications by use of a temperature adjusting circuit. Moreover, similar improved distributed reflection type lasers each of which uses a sample diffraction grating structure, or a super-structure diffraction grating structure, or the like in the diffraction grating part require further complicated control circuits.
On the other hand, in development is a multi-wavelength distributed feedback laser-array device of an optical multiplexer integrated type in which distributed feedback lasers each having a different wavelength are arrayed in a transverse direction or in a longitudinal direction and optical outputs of the lasers are multiplexed in a single location. In this device, for distributed feedback lasers, one that is of a λ/4 phase-shift type or of a gain coupling type each of which controls fabrication difference of the mode phase is adopted, and hence basically stable single-mode operation can be obtained. However, the laser has intrinsic problems which require improvement in that laser channels must be switched over every time the emission wavelength is varied for a wavelength range of a few nm and a response speed at the time of switching the wavelength range is limited to a few msec because the wavelength needs to be fine tuned by controlling an operating temperature of the laser.
Further, although no example of realization has been achieved until now, there can easily be conceived a way of making an array of the above-mentioned distributed reflection type lasers that have wider tunable wavelength widths than the distributed feedback laser to effect expansion of the wavelength range. However, this configuration has an intrinsic problem that, since each distributed reflection type laser has a plurality of control terminals, the number of control terminals is multiplied by the number of arrayed lasers described in IEEE Photonics Technology Letters, Vol. 12, No. 3, pp. 242–244, March 2000.
In the above-mentioned conventional tunable laser, a highly complicated wavelength control circuit is indispensable.